2.4    Empirical Formula and Molecular Formula

Percentage composition by mass

Empirical formula

• Empirical formula is a chemical formula that shows the simplest ratio of the atoms that combine to form a molecule
• Steps to find empirical formula:
  1. Find the mass of each element
  2. Find the number of mole of  each element (divide by its   Ar).
  3. Find the simplest ratio (divide by the smallest number).
  4. Construct the empirical formula using the simplest

[ If a decimal or fraction exists, round up or eliminate the fraction ] [ Never assume a formula ]

• Some facts:
  1. The formula for an ionic compound is always its empirical formula
  2. The empirical formula and molecular formula for simple inorganic molecules are often the same
  3. Organic molecules have different empirical and molecular formula

Molecular formula

• Molecular formula is a chemical formula that shows the actual number of atoms that combine to form the compound
• In order to deduce the molecular formula of a compound, we need to know:
  1. the relative formula mass of the compound
  2. the empirical formula of the compound

Principle of  conservation of  mass

• Mass is neither created nor destroyed during a chemical reaction. Therefore the total mass of the reactants is equal to the total of the products in a closed system.
• For example, the total mass of iodine in the reactants is equal to the total mass of iodine in the products
• This can be used to solve problems in calculating the empirical formula